The present invention relates to the field of analog-to-digital converters. More specifically, the present invention relates to multislope, continuously integrating analog-to-digital converters.
Analog-to-digital (A/D) converters are known. One conventional type of A/D converter is the dual slope integrating converter. Dual slope converters are switched between an input voltage and a reference voltage at controlled times. The value of the input voltage is determined from the time required to discharge the integrator to zero voltage with the reference voltage. Dual slope converters require intermittent switching, which results in aliasing of noise components into the converter""s signal band. Also, quantization errors, like other noise, accumulate over repeated conversion cycles.
Multislope A/D converters are similar to dual slope converters, with the exception that reference voltages of varying magnitudes are applied to decrease measurement time, without decreasing resolution. The stability of multislope converters, however, is dependent upon the absence of an input signal during the run-down period. Also, the resolution for a multislope converter is limited by total signal input energy.
A third type of conventional A/D converter is the continuously integrating A/D converter. A continuously integrating converter maintains the input signal switched to an integrator. This configuration results in less noise. However, a disadvantage is that resolution is limited by bandwidth requirements for conversion of high speed signals.
The conventional A/D converters discussed above accumulate errors during use. For example, the precision of the components such as resistors, capacitors, etc. varies, which can lead to errors in the conversion process. These types of errors are typically corrected by calibration. However, conventional calibration requires control logics dedicated to the calibration process, which increases the cost of conventional A/D converters. Also, conventional calibration does not adequately compensate for errors as they occur during operation of A/D converters.
Therefore, a need exists for an analog-to-digital converter having a high resolution over a wide dynamic range, with low noise effects. A further need exists for an analog-to-digital converter that can be calibrated without adding undue cost.
The present invention satisfies the above needs and achieves other advantages not present in conventional devices.
According to a first aspect of the invention, an analog-to-digital converter comprises a first switch coupled to a first large reference voltage, a second switch coupled to a second small reference voltage, a third switch coupled to an input voltage, an integrator operably coupled to the first, second, and third switches, a comparator coupled to the integrator, and control logic coupled to the first, second, and third switches. The second reference voltage has a polarity opposite to the first reference voltage.
According to the first aspect, the resolution of the analog-to-digital converter can be increased according to the second, smaller, reference voltage, and the dynamic range can be increased according to the first, larger, reference voltage. In addition, the use of reference voltages of opposite polarity eliminates the need for an offset of the input voltage to maintain an input signal of constant polarity.
According to a second aspect of the invention, the analog-to-digital converter is operable in either an analog-to-digital conversion mode, or a calibration mode. During a calibration process, a calibration factor is calculated for application during a conversion process. When applied to the conversion process, the calibration factor corrects for errors in the conversion process.
According to the second aspect, the calibration mode is similar in operation to the conversion mode. Therefore, the calibration factor accurately reflects the errors that occur during the conversion process. In addition, control of the conversion and calibration processes is simplified.
Other aspects and advantages of aspects of the invention will be discussed with reference to the drawings figures and to the detailed description of the preferred embodiments.